It is desirable to provide an effective process to increase the yield of gasoline (naphtha) in catalytic cracking units. More particularly, catalytic cracking of oil is an important resid hydrotreating unit process which is used to produce gasoline and other hydrocarbons. During catalytic cracking, a feedstock, which is generally a cut or fraction of crude oil, is cracked in a reactor under catalytic cracking temperatures and pressures while in the presence of a catalyst in order to produce more valuable, lower molecular weight hydrocarbons. Gas oil, which is usually used as a feedstock in catalytic cracking, typically contain from 55% to 80% gas oil by volume, having a boiling range from 650.degree. F. to 1000.degree. F. and less than 1% Ramscarbon by weight. Gas oil feedstocks usually contain less than 5% by volume naphtha and lighter hydrocarbons having a boiling temperature below 430.degree. F., from 10% to 30% by volume diesel and kerosene having a boiling range from 430.degree. F. to 650.degree. F., and less than 10% by volume resid having a boiling temperature above 1000.degree. F.
Known processes catalytically crack virgin unhydrotreated, low sulfur resid as well as deasphalt, subsequently hydrotreat, and catalytically crack high sulfur resid. Furthermore, such prior art processes produce hydrogen-rich asphaltenes which are difficult and expensive to handle, process, and melt (liquefy) at relatively low temperatures. These asphaltenes cannot be used as solid fuel, are difficult to blend into fuel oils, and are not generally usable and desirable for asphalt paving or for use in other products.
Refiners have used deasphalting processes to fractionate low sulfur reside ("LSR") and to enhance the processing of the resulting fractions. Typically, the low sulfur resid is fractionated into an oils fraction and a heavy fraction including resins and asphaltenes. The oils fraction is a desireable feed for a catalytic cracking process because it contains relatively small amounts of metals, nitrogen, and refractory coke-forming compounds. Typical catalytic cracking yield from the oils fraction are similar to those obtained from virgin gas oils. The small amounts of metals and refractory compounds allow large amounts of the oils fraction to be processed in a catalytic cracker ("FCCU") or in a fixed bed hydrotreater followed by catalytical cracking.
The heavy fraction is difficult to process in order to obtain high yields of the lighter, more valuable products. The heavy fraction has a high Ramscarbon content so that the liquid yields from coking this fraction are modest. Therefore, the heavy fraction is often blended into paving asphalt, but its properties make it a poor blending stock. To overcome this blending problem, a portion of the valuable oils fraction is blended with the heavy fraction in order to blend into the asphalt. The heavy stock may also be blended into low value fuel oils, but the poor properties of the heavy fraction limit the amount of that fraction which can be disposed of by this means. Another means of utilizing the heavy fraction are to use it as a low value fuel.
Low sulphur resids are relatively poor feedstocks for a resid hydrotreating processes. In resid hydrotreating, the resid is subjected to a high temperature process in the presence of hydrogen and a hydrogenation catalyst. The objectives of these processes, such as a conventional hydrotreating process, are to remove sulfur, nitrogen, and metals, and to saturate olefins and aromatic compounds. In addition, the resid boils above 1000.degree. F. where it is converted to lighter products, which can be subsequently upgraded in other refining units.
High sulfur resid ("HSR") is significantly more reactive than low sulfur resids in hydrotreating processes. This quality limits the usefulness of the low sulfur resids in hydrotreating processes.
Carbonaceous solids are produced as a by-product of the reaction which occurs during ebullated bed hydrotreating (expanded bed hydrotreating). The ebullating hydrotreating catalyst fines serve as a nucleus and center for carbonaceous solids formation. The situation becomes even more aggravated when two or more hydrotreating reactors are connected in series, as they are in many, if not most, commercial operations. Solids formed in the first reactor not only form nucleation sites for solids growth and agglomeration in the first reactor, but are carried over with the hydrotreated product oil into the second reactor, etc., forming even larger solids growth and agglomeration in the process.
The concentration of carbonaceous solids increases at more severe hydrotreating conditions, at higher temperatures, and at higher resid conversion levels. The amount of carbonaceous solids is dependent on the type of feed. Hence, resid conversion is limited by the formation of carbonaceous solids.
Over the years, a variety of processes and equipment have been suggested for use in various refining operations, such as for upgrading oil, hydrotreating, reducing hydrotreated solids, and catalytic cracking. Typifying some of these prior art processes and equipment are those described in U.S. Pat. Nos. 2,382,282; 2,398,739; 2,398,759; 2,414,002; 2,425,849; 2,436,927; 2,962,222; 2,884,303; 2,900,308; 2,981,676; 2,985,584; 3,004,926; 3,039,953; 3,168,459; 3,338,818; 3,351,548; 3,364,136; 3,513,087; 3,563,911; 3,661,800; 3,766,055; 3,798,157; 3,838,036; 3,844,973; 3,905,892; 3,909,392; 3,923,636; 4,191,636; 4,239,616; 4,290,880; 4,305,814; 4,331,533; 4,332,674; 4,341,623; 4,341,660; 4,354,922; 4,400,264; 4,454,023; 4,486,295; 4,478,705; 4,495,060; 4,502,944; 4,521,295; 4,526,676; 4,592,827; 4,606,809; 4,617,175; 4,618,412; 4,622,210; 4,640,762; 4,655,903; 4,661,265; 4,662,669; 4,692,318; 4,695,370; 4,673,485; 4,681,674; 4,686,028; 4,720,337; 4,743,356; 4,753,721; 4,767,521; 4,769,127; 4,773,986; 4,808,289; and 4,818,371. These prior art processes and equipment have met with varying degrees of success.